1. Field of the Invention
The present invention relates to a light source device for generating extreme ultra violet (EUV) light by irradiating a target with a laser beam. Furthermore, the present invention relates to exposure equipment using such light source device.
2. Description of a Related Art
With miniaturization of the semiconductor process, the photolithography makes rapid progress in miniaturization, and, in the next generation, micro-fabrication of 100 nm to 70 nm, further, micro-fabrication of 50 nm or less will be required. For example, in order to fulfill the requirement for micro-fabrication of 50 nm or less, the development of exposure equipment with combination of an EUV light source for generating light having a wavelength of about 13 nm and a reduced projection catoptric system is expected.
EUV lithography is a kind of photolithography which uses extreme ultraviolet light having a wavelength range of 10 nm to form a circuit by forming a mask image of patterns of a semiconductor circuit onto a resist coated on a semiconductor wafer in the reduced projection catoptric system. In the exposure equipment to be used for EUV lithography, it is assumed that the throughput is 80 sheet/hour and the resist sensitivity is 5 mJ/cm2, and, in the case of using the constitution of the optical system that is conceived at present, the EUV light source of about 10W to 1000W is required.
As the EUV light source, there are three kinds of light sources, that is, an LPP (laser produced plasma) light source using plasma generated by irradiating a target with a laser beam, a DDP (discharge produced plasma) light source using plasma generated by discharge, and an SR (synchrotron radiation) light source using orbital radiation. Among them, the LPP light source has advantages that extremely high intensity near black body radiation can be obtained because plasma density can be considerably made larger, light emission of only the necessary waveband can be performed by selecting the target material, and there is no structure such as electrodes surrounding the light source and an extremely large collection solid angle of 2π sterad can be ensured because it is a point source having substantially isotropic angle distribution, and therefore, the LPP light source is thought to be predominant as a light source for EUV lithography requiring power of about 10W to 1000W.
In the LPP light source, in the case where a solid material is used as a target which is irradiated with a laser beam for generating plasma, the heat generated by the laser beam irradiation is conducted to the periphery of the laser beam irradiated region, and the solid material is melted on the periphery thereof. The melted solid material is released in large quantity as debris having a diameter more than several micrometers, and that causes damage to a collective mirror to reduce the reflectance thereof. On the other hand, in the case where gas is used as the target, debris is reduced but the conversion efficiency from the power supplied to the laser oscillator into the power of EUV light is reduced.
By the way, since the LPP light source is a point source or a set of point sources, it is necessary to collect the light diverged from the LPP light source by using the collective mirror so as to output light available to the EUV lithography. Here, in luminous transmission of point source light, there exists a principle that etendue is always constant. The etendue is a quantity defined by the product of a luminous area and a spreading angle (solid angle). When the light source side etendue (the product of the light source area and the diverging solid angle) is larger than the illuminated region etendue (the product of the illuminated region and the solid angle of illumination light), the ratio of the luminous that can not be taken in the illuminated region is increased, and accordingly, the light source side etendue is needed to be suppressed smaller than the illuminated region etendue. Since the EUV light is diverging light, in order to suppress the etendue smaller, the size of plasma as the light source should be made sufficiently small. For example, in order to collect the light in a range from the light source to the solid angle π, the diameter of plasma is needed to be less than about 0.5 mm.
Conventionally, in order to generate plasma in the LPP light source, the development has been performed by using a 1.5W class LD excitation YAG laser. This YAG laser exhibits pulse duration of several nanoseconds and the wavelength of the laser beam to be used is in the 1 μm range. On the other hand, the generation process of the plasma exceeding several tens of thousands degrees is developed on a scale of picoseconds (10−12 seconds). In the case where the plasma density is small at the initial time of irradiation of the laser beam, the laser beam subsequently passes through without turning molecules and atoms in the target into a plasma state sufficiently. On the contrary, in the case where the plasma density is too large, the laser beam is intercepted by the plasma on the side irradiated with the laser beam, the plasma having sufficient volume can not be generated. Accordingly, there is a suitable range for the target gas density.
When the YAG laser is used, it is necessary to allow the laser beam to interact with the target gas having rather large density for efficient absorption of the laser beam. Therefore, it is necessary to irradiate with the laser beam the gas having large density near the orifice of the nozzle. However, there are problems that the high-powered YAG laser is poor in collectivity because multiple modes exist in the transverse mode, which becomes deteriorated further because the nonuniformity of the glass medium increases due to heat generated during operation, and, as a result, the irradiation efficiency to the target is reduced.